1. Field of the Invention
The present invention relates to perfecting sheet rotary offset presses or printing machines, and more particularly to machines capable of performing offset printing on both the top and back surfaces of a sheet respectively in one color or in a plurality of colors through a single sheet feeding operation.
2. Description of the Related Art
Conventionally, among sheet rotary offset presses, there are known a so-called reversal perfecting press, a blanket-to-blanket perfecting web offset press, as well as other types of presses.
An example of the reversal perfecting press is disclosed in Japanese Patent Publication No. 1-56672 which comprises a back surface printing section, a top surface printing section, a sheet reversing section, and so on. After the back surface of a sheet has been printed in a plurality of color by the back surface printing section, the sheet is reversed by the sheet reversing section to transfer the sheet to the top surface printing section which then provides multi-color printing on the top surface of the sheet.
The blanket-to-blanket perfecting web offset press is disclosed in Laid-open Japanese Patent Application No. Here a plurality of blanket cylinders and impression cylinders, each provided with a rubber blanket adhered thereon, are alternately arranged in the horizontal. A top portion of each blanket cylinder and a bottom portion of each impression cylinder are respectively provided with a plate cylinder, an ink feeding mechanism, a dampening arrangement, and so on to constitute a printing unit. Further, each of the blanket cylinders and impression cylinders is provided with a gripper such that multi-color printing is performed on both surfaces of a sheet while the sheet is being transferred.
An example of other types of presses is disclosed in Japanese Patent Publication No. 3-21346. In this press, two pairs of laterally arranged plate cylinders and rubber cylinders are combined with a common impression cylinder, which has a diameter three times larger than that of the cylinders of the plate cylinders and rubber cylinders, and is provided with a sheet gripper, so as to constitute a five-cylinder-two-color printing unit. Then, two sets of printing units are alternately arranged in the vertical direction such that the positional relationship of the plate cylinders and the rubber cylinders with respect to the impression cylinder is defined in an alternate manner in the horizontal direction. The respective impression cylinders are coupled such that a line connecting the axes of the impression cylinder presents a zig-zag line. A sheet feeder unit is coupled to an impression cylinder located at the lowermost position, while a take-off unit is coupled to an impression cylinder at the topmost impression cylinder, so as to achieve four-color printing on both top and back surfaces of sheets in two-color unit.
However, the reversal perfecting press has a drawback in that due to the fact that it employs a gripper for gripping the trailing edge portion of a sheet tranferred by the sheet reversing section its mechanism behaves rather violently, the printing speed is lower than a one-surface press, and the registering accuracy for printing is easily degraded. In addition to this, when the size of the sheet is to be changed, a great deal of time is required to alter the gripper position on the trailing edge of the sheet. The size of sheets processed by the prior art press is limited to a certain range. Furthermore, since the prior art press requires two sheet feeding cylinders and two reversal cylinders for the reversing mechanism, the length of the whole machine is extremely large.
The blanket-to-blanket perfecting web offset press in turn simultaneously prints on both surfaces of a sheet, so that, a printed sheet may possibly touch the blanket cylinder, on which ink is attached, to cause a stained sheet. Also, doubled printing may occur due to the structural defects. More specifically, a possible error in the thickness of blankets may result in a delicate difference in the cylinder arrangement dimension of the two blanket cylinders, which may in turn lead to doubled printing. This type of perfecting press therefore requires countermeasures to avoid such defective printing. Further, since the blanket-to-blanket perfecting web offset press does not have an impression cylinder which serves as a reference plane for applying a printing impression, printed sheets produced thereby is disadvantageously lacking in sharpness.
The perfecting press described in the above-mentioned Japanese Patent Publication No. 3-21346 is suitable only for printing on sheets of small sizes. It is not therefore practical to modify this perfecting press so as to print on sheets of relatively large sizes, in consideration of the size of printing plants, because of the height of the press which would be unacceptably increased if modified for sheets of larger sizes.